1. Field of the Invention
The present invention relates generally to the processing of signals in code division multiple access (CDMA) wireless systems such as CDMA cellular radiotelephone systems. More particularly, the present invention relates to the equalization and filtering by an adaptive filter (AF) of a CDMA signal that may have been corrupted by distortion, noise and interference during transmission from the base station to the user handset.
2. Description of the Related Art
Code division multiple access or CDMA techniques are gaining popularity in current and next generation wireless networks as an efficient way of improving capacity, both with regard to the number of users and the achievable bit rates. Examples of CDMA wireless networks include those that operate according to the IS-95 standard, currently in use in the United States for mobile cellular telephone networks, and those networks using successor technologies such as CDMA 2000 and specifically the nearest term successor technology, CDMA 2000-1x. Another CDMA technology gaining acceptance is WCDMA. This background discussion and the subsequent discussion of implementations of the invention make specific reference to the structure and implementations of CDMA 2000-1x. This is not intended to be limiting. The invention described below can be applied to the various versions of CDMA 2000, to various versions of WCDMA and to other successor technologies.
Because CDMA signals simultaneously occupy a given frequency band and an arbitrarily long time interval, CDMA systems use codes that identify users to achieve multiplexing (code division) of users. Different base stations (corresponding for example to different cells) need isolation within the CDMA system so that a receiver can readily distinguish between base stations. Generally the isolation between base stations is accomplished with another code, different from the code that identifies users. As such, the CDMA 2000-1x system, like its predecessor IS-95, uses orthogonal codes (e.g., Walsh codes) to identify or isolate user subchannels and system control subchannels, and uses maximal length sequences (e.g., pseudo-noise “PN” codes) to identify or isolate different base stations. The service provider reuses a given frequency band within its network by employing it in cells that are spaced apart by a sufficient distance so that the cells do not unduly interfere with each other's transmissions when the PN codes employed by adjacent cells are different. This difference can be as simple as a significant time displacement between two copies of the same basic PN code. This is the approach employed in the CDMA 2000 system for example.
WCDMA operates similarly. In WCDMA, the functions that separate individual user subchannels are called orthogonal variable spreading functions (ovsf). Where the terms “Walsh function” or “Walsh code” is used in the following discussion, it is intended to include the orthogonal variable spreading functions as well as other similarly used orthogonal functions or codes.
A CDMA base station constructs its downlink signal by assigning each subchannel an identifying Walsh code or other orthogonal code and using that orthogonal code to spread the subchannel's signal. Unique orthogonal codes are assigned to the user subchannels so that a receiver can select its subchannel from the base station broadcast and reject the other subchannels using the code. The base station also modulates the user's signal with a PN code (and/or a time shifted version of a common PN sequence) specifically identifying that base station on the network. Each base station uses the first subchannel as a pilot channel by sending a known data stream, generally consisting of all 1's, over the channel. The pilot channel is used by terminal receivers to identify and lock onto the signal from a desired base station. A second of the base station's subchannels (the sync channel) is employed to transmit control information to the receiver terminals. Most of the bits transmitted on the sync channel are predictable. The other base station subchannels contain (from the receiver viewpoint) more or less random user bits.
Interference sources of concern to receivers in a CDMA system include multipath arriving within the time window (e.g., 14 μs) used by a receiver for observing signals. Other interference sources include the downlink signals from base stations other than the one the receiver is using for communication. The Walsh codes and PN codes are designed to prevent interference between subchannels and with other base stations. Some interference is inevitable. Although separate PN sequences are nearly (or are forced to be) orthogonal to each other, the orthogonality condition requires an integration time as long as the code. Generally, the symbol period consists of a number of chips that represent the period of the Walsh code. The symbol period is often 64 chips for IS-95 and CDMA-2000 systems, and can be between four chips and 512 chips for various CDMA systems. Therefore, the typical observation intervals are too short relative to the codes to achieve complete orthogonality relative to physically adjacent base stations to prevent interference from such base stations. This interference is reduced by the code properties but is dependent on the power of the signals from other base stations at the receiver, which can be higher than the power from the desired base station when fading is present.
The simultaneous presence of all the subchannels and the use of both a subchannel code (orthogonal or Walsh code) and a base station code (e.g., PN code) make it difficult to use equalization techniques to increase the capacity of existing CDMA networks, including CDMA 2000 networks. Most of the work to date on CDMA reception involves the use of rake receivers to mitigate multipath and to improve reception when multiple user interference occurs. Plural, generally independent, receiving channels known as rake fingers are provided in a rake receiver to improve, for example, processing of the signals associated with multipath and signals received from different base stations.
Rake receivers generally are made up of a searcher and a combiner, which includes the rake fingers. The searcher utilizes the pilot channel to locate, in time, a unique strong signal for each rake finger. The searcher employs the pilot subchannel to identify path delays, amplitudes and phases and provides that information to the respective rake fingers, which use the information to better recover the different multipath contributions. Because the number of rake fingers is limited, the rake receiver cannot assign a rake finger to all significant multipaths. A typical rake receiver tracks only three paths, although suggestions have been made to track up to twelve paths. The need to track additional multipath contributions is most prominent in urban environments. There are other shortcomings to rake receivers, such as performance dependent on the separation of path delays.